The present invention relates to a tracking servo system for a magnetic recording and reproducing apparatus using a flexible magnetic disk.
The magnetic recording and reproducing apparatus using a flexible magnetic disk, i.e. a flexible disk unit has features of low cost, small size and easy maintenance as compared with an interchangeable or fixed disk unit using a hard or rigid magnetic disk, and also has the advantages of short access time and random access capability over a magnetic tape memory unit such as a cassette tape. Therefore, the flexible disk drive unit has been widely used as a simple external storage in minicomputer and microcomputer systems.
The fixed disk unit using a hard magnetic disk generally employs a tracking servo system, and the recording track width or track pitch on the magnetic disk is chosen to be small in order to obtain a large track density and thereby to increase the storage capacity. On the other hand, in the conventional flexible disk drive unit, since the flexible magnetic disk intrinsically involves a large eccentricity of the track and a large variation in track diameter, a tracking servo system will not successfully respond. In fact, most of the conventional flexible disk drive units employ no tracking servo system because they have been developed under the requirements of low cost. Therefore, the conventional flexible disk drive unit has a relatively small storage capacity. However, the processing ability of microcomputers or the like is dramatically advancing owing to the progress of semiconductor technologies, and the flexible disk drive unit is now requested to have an increased storage capacity.
In the conventional tracking servo system for the fixed disk unit using a hard magnetic disk, one entire surface of a disk or a plurality of disks secured on the same axis is used exclusively for a tracking servo. FIG. 1a shows the pattern of servo tracks used in such a conventional tracking servo system. As shown in the figure, the disk surface used for tracking servo has the records of servo tracking signals f.sub.1 and f.sub.2 arranged alternately to form servo tracks. With the center of a gap 201 of a servo magnetic head 2 being located at the boundary of tracks N and N+1, the head 2 reproduces the signals f.sub.1 and f.sub.2 in the same amplitude P.sub.0, as is apparent from FIG. 1b. If the servo head 2 moves toward the track N+1, the amplitude Pf.sub.1 of the reproduced tracking signal f.sub.1 increases while the amplitude Pf.sub.2 of the reproduced tracking signal f.sub.2 decreases. Conversely, if the servo head 2 moves toward the track N, the amplitude Pf.sub. 2 of the signal f.sub.2 increases while the amplitude Pf.sub.1 of the signal f.sub.1 decreases. The reproduced tracking signals f.sub.1 and f.sub.2 are separated using a band-pass filter means or the like, and the servo head 2 is moved in accordance with a difference in amplitudes between the separated signals f.sub.1 and f.sub.2 so that the servo head 2 follows the boundary of the tracks N and N+1. Data is recorded on a disk surface different from the disk surface used for the tracking servo by means of a data head which moves in interlocking relation with the servo head 2. The servo head 2 is used solely for reading out the tracking signals f.sub.1 and f.sub.2 which have been recorded in the manufacturing process of the disk. The recorded signals are not erased or rewritten in usual cases.
The flexible magnetic disk is formed of a disk-shaped Mylar sheet with a magnetic medium coated on each of both surfaces thereof, and the sheet is enclosed in a soft case about 1 mm thick. If one surface of the disk is used exclusively for tracking servo, only the other surface is available for recording data, resulting in a poor efficiency of utilization. Unlike the fixed disk, the flexible disk detachable from the recording and reproducing apparatus and having exposed magnetic surfaces has the risk of demagnetization of the recorded tracking signals due to erroneous handling operation. Therefore, it is preferable that the tracking signals can be easily or freely rewritten. Accordingly, the conventional tracking servo system explained in conjunction with FIG. 1a is not suited for use in the recording and reproducing apparatus using a flexible disk.
There is known a system in which both data and servo tracking signals are recorded on the same magnetic surface of a magnetic disk in such a manner that each data track is divided into several sectors and a tracking signal is recorded between the sectors. However, in the case of the flexible magnetic disk, the number of sectors for each track is generally small and varies significantly depending on a data format used. For example, the standard 8-inch diameter flexible disk can have any of 8, 15 and 26 sectors. Therefore, this system is also unsuited for use in the recording and reproducing apparatus using a flexible disk unit.
There is also known, a system in which both data or the like and tracking signals are recorded on the same track in superimposed fashion. This system is employed in video tape recorders for home use and it is disclosed in detail in U.S. Pat. No. 4,297,733. This system will now be described with reference to FIGS. 2a and 2b FIG. 2a shows a magnetic surface of a magnetic tape in which superimposed video and tracking signals are recorded without any guard band. The video signal is azimuthally recorded so that the video signal recorded on a given track is protected from interference by signals recorded on the adjacent tracks. A magnetic head 4 used for both recording and reproducing operates to record the video and tracking signals in superimposed fashion on the magnetic tape and to reproduce the signals therefrom. The tracking signal is selected to have a frequency band which is sufficiently separate from that of the video signal, and these signals are separated using a filter means. Usually, four kinds of frequency are cyclically used for the tracking signals. In recording, video and tracking signals are recorded on the magnetic tape in superimposed fashion by the magnetic head 4 without carrying out the operation of a tracking servo. In reproducing, only the tracking signal components are extracted from the reproduced complex signal by the filter means so that it is used for a tracking servo. When the magnetic head 4 is located at the center of a track N, only the tracking signal f.sub.2 is obtained as shown in FIG. 2b. If the magnetic head 4 deviates toward a track N+1, the amplitude Pf.sub.3 of the tracking signal f.sub.3 increases, and if the head deviates toward a track N-1, the amplitude Pf.sub.1 of the tracking signal f.sub.1 increases. In practice, the tracking signals f.sub.1, f.sub.2 and f.sub.3 have respective frequencies lower than the video signal frequency. Therefore, even if a gap 401 of the magnetic head 4 is precisely located on the track N, the signals f.sub.1 and f.sub.3 are not nullified due to the magnetic fluxes leaking from the adjacent tracks. Accordingly, a tracking servo system is designed such that the magnetic head 4 is moved toward the track N+1 in proportion to the amplitude PF.sub.1 of the reproduced signal f.sub.1 and toward the track N-1 in proportion to the amplitude Pf.sub.3 of the reproduced signal f.sub.3 so that the magnetic head 4 is urged to a position where the signals f.sub.1 and f.sub.3 have the same amplitude and the video signal recorded on the track N can be reproduced correctly.
In the example shown in FIGS. 2a and 2b, it is difficult to carry out the operation of tracking control during recording. For example, the recording and reproducing magnetic head 4 must operate to record a video signal and a new tracking signal f.sub.2 while reproducing the tracking signals f.sub.1 and f.sub.3 pre-existing on the adjacent tracks. This operation is generally very difficult. An additional head may be provided for reproducing the tracking signals on two tracks opposite to any given track to carry out the operation of tracking servo by the reproduced signals. However, though a new tracking signal must be recorded on the given track for every recording operation, this fact will result in a possibility that the variation in position of the track due to the tracking servo error is accumulated with the repeated recording operations and, thereby provide a large variation in the track position. In the case of the video tape recorder, this does not pose a serious problem since a large amount of video information is continuously recorded and continuously reproduced. But, in the case of the flexible disk where a signal is frequently recorded on and reproduced from a selected track in a random access fashion, the track position needs to be set accurately.